Polarized electrons produced by atomic beam apparatus - hyperfine structure intervals of alkali atom

Hughes, V. W.

Long, R. L., Jr.

Raith, W.

English

NASA Technical Reports Server

I 
' GPO PRICE $ 
1 .  
- 1  
I 
ff 653 July 65 
&Atomic Beam Source of Polarized E l e c m  
V,W.HUGHES, R.L.LONG, Jr. and W,RAITH 
\Gibbs Labratory, Yale U-niversity 
C A  (2 C / q ( j l  
f 6 31AL 3 r New Haven, - c _  Conn,ectm) 
At least four methods of obtaining polarized electrons 
have been investigated recently: 1) photoelectric emission from 
a ferromagnet, 2) field emission from a ferromagnet, 3) optical 
pumping techniques, 4) atomic beam techniques. 
Measurements of the polarizations of photoelectrons 
from a single crystal of nickel and from thin films of iron, 
nickel and cobalt(2) by use of Mott scattering have shown that 
the polarizations are zero within the experimental errors of 3% 
to 5%. Similarly, the polarization of field-emitted electrons 
from magnetized iron has been found (394) to be less than 0.2$, 
Optical pumping techniques for producing polarized electrons, 
which are being studied in several laboratories, are based on the 
possibilities of producing polarized alkali atoms by use of polar- 
ized resonance radiation(5) and of producing polarized electrons 
in exchange collisions of  electrons with polarized alkali atoms. ( 6 )  
Polarized electrons have been produced successfully ( 7 )  
by an atomic beam method in which polarized alkali atoms are 
selected by deflection in an inhomogeneous magnetic field(8) and 
the polarized electrons are then obtained by photoionization of 
the polarized atoms, The energy level *of  the hyperfine 
structure levels of the ground state of potassium in a magnetic 
'1 
https://ntrs.nasa.gov/search.jsp?R=19650019781 2020-03-17T02:04:16+00:00Z
* * r :  t 
- 2 -* 
field is shown in figure 1. 
geneous magnetic field atoms in states with mJ = +1/2 (mJ = mag- 
netic quantum number for electron spin) can be separated from 
atoms in states with mJ = -1/2. 
ciently strong in the region where photoionization occurs, the 
photoelectrons will have a large polarization since the photoioni- 
zation process is predominantly an electric dipole transition. 
The electronic polarization for potassium of the group of states 
having mJ = +1/2 in a strong magnetic field is shown as a func- 
tion of magnetic field in figure 2. 
By deflection in a strong inhomo- 
1 
If the magnetic field is suffi- 
-., . 
The apparatus constructed at Yale for the production 
and anlysis of a source of polarized electrons is shown schemati- 
cally in figure 3. The oven has 30 circular apertures each 0.025 
cm in diameter and hence a total aperture of 0.014 em . 
homogeneous magnetic field is provided by a six-pole permanent 
magnet (see figure 4) with a pole t i p  field of 9000 gauss. 
1. L * 
2 The in- 
In an 
idealized six-pole configuration the magnitude of the magnetic 
field H is given by: ( 9 )  
H = H, ( )2, independent of $ 
in which Ho is the pole tip field, a is the radial coordinate of 
the pole tip, and (r,jd) are cylindrical coordinates. 
on a magnetic dipole moment is 
The force 
1 .  
- 3 -  
in which p is the component of the magnetic dipole moment in 
the radial direction. If p is negative, the particle will 
experience a force towards the axis and will execute simple 
harmonic motion with a wavelength 
r -
in which v is the velocity of the particle, m is the mass, and 
wo is the Bohr magneton. 
be deflected away from the axis. 
p (mJ = +1/2) are focussed into the photoionization region. 
axial magnetic field of about 400 gauss is applied in the photo- 
ionization region by Helmholtz coils exterior t o  the vacuum sys- 
tem. Ultraviolet light is focussed onto the atomic beam and the 
photoelectrons are extracted from the photoionization region, 
which is at a potential of -120 kv, and then accelerated to 
ground potential. 
plished by Mott scattering after the polarization has been 
changed from longitudinal to transverse by deflection in an 
If p is positive, the particle will 
Particles with negative 
An 
Analysis of the polarization is to be accom- 
electric field. (10) 
The design characteristics of the polarized electron IL. 
source with potassium atoms are listed in Table I. The appara- 
tus  has been constructed and is presently under test. 
f 
4 
; f 
- 4 -  
TABm I 
Design Characteristics of Potassium 
Polarized Electron Source. 
F ~ U  from oven = 4 x 1018 atoms/sec 
Fraction accepted by magnet = 5 x 
Fraction in mJ = +1/2 states = 0.5 
Fraction transmitted by magnet 
Polarized atoms into ionizer = 8 x 10 14 /sec 
Ionization efficiency = 3 x 10 
Accelerated electron beam = 2 x 1O1’/sec o r  3 x 10- 8 amp 
Electron polarization = 9% 
= 0.8 
-4 electrons/atom 
The choice of alkali atom is determined primarily by 
its photoionization cross section (see figure 5) ani! by its hyper- 
fine structure interval. (See Table 11) From figure 5 it is seen 
that lithium has a particularly large photoionization cross sec- 
tion, and xenon light sources in the appropriate wavelength range 
are now available. 
804 Mc/s so that the static magnetic field required in the photo- 
Furthermore the hfs interval Av for Li7 is 
ionization region in order t o  obtain an electron polarization of 
go$ is only about 500 gauss. Hence lithium may be the most favor- 
able atom to use for a polarized electron source. Although 
cesium has a reasonably large photoionization cross section over 
a useful wavelength range, its hfs interval is so large that an 
inconveniently large static magnetic field is required in the 
- 5 -  
photoionization region. Our initial studies are being done with 
potassium because the potassium atom is easily detected with a 
hot wire surface ionization detector (whereas detection of lithium’ 
requires oxygenation of the wire) and because a mercury light 
source can be used for photoionization of potassium. 
TABLE I1 
Hyperfine Structure Intervals 
Alkali 
Atom 
Isotopic 
Abundance HFS Interval, Av 
(percentage ) (in Mc/s) 
Li6 
Li7 
Na23 
K3’ 
7.5 
92.5 
100 
93 
7 
73 
27 
100 
228 
804 
1772 
462 
254 
3036 
6835 
9193 
Polarized electrons could be used to study the 
electron spin dependence of atomic collisions cross sections 
in a crossed electron-atom beam experiment. 


Atomic beam source of polarized electrons

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server